I am also an aviation photographer (spotter) and also had a hard time deciding on my super tele.I own the 70-200 f/4 and was considering the 100-400L but I've seen a lot of soft shots and complaints from people who own them. I was leaning towards the versatility and the "IS" which is of course useful in our case. Yet the 100-400L new was 1700$ and ended up buying the 400L + some accessories. The 300 f/4L IS + 1.4 II was something I didn't look at too much because I do believe it was slightly more expensive than the 400L and the 1.4x was another 300$ which would turn out to be even more expensive than the 100-400L. Also, since I already have a 200mm lens I found that their won't be a huge difference to have 100mm extra.

Although, sometimes I feel as if I made the wrong decision purchasing the 400L because on my cheap body (Rebel XS/1000D) the results aren't superb. The lens seems soft or oof most of the time (probably camera fault) with all possible manual settings. With speeds under 1/100 it's also soft even if I have good stability or support. I don't see it performing badly on the 50D as i've seen great shots from the 40D and above.

Depending on your spotting locations and the type of shots you want you should consider:- Do I want the most zoom with the best IQ?- 400L- Do I want more versatility and the possibility to have 420mm f/5.6L with some IQ loss? - 300L + 1.4- Do I want complete versatility with not as good IQ (even with 1.4x). - 100-400L

My 400mm f5.6 is sooooo sharp and I can highly recommend it. If you are cropping heavy then it would appear you need the reach rather the flexibility of a zoom / lens and t.c. combination. I don't particulary miss IS as need a high shutter speed for the photography I do (birds and birds in flight) if your are shooting planes in flight then I would assume similar.If you want to see how sharp the 400 f5.6 is have a look at my bird photos on flickr.

I own both the 400/5.6 and the 300/4. I also have the 70-200/2.8 II + 2x III and have owned the 100-400 in the past. For your purposes (aviation) the 400/5.6 would seem to be a no brainer. You will already be using a high shutter speed so you won't miss the lack of IS.

Contrary to what you may read elsewhere the 300/4 takes extenders very poorly. I tried it once and was so disappointed by the results that I never tried it again. That being said, it is a great lens. If you can get close to your subject then it has a great macro capability. On the aviation side it could be used for interesting closeups of aircraft on the ground.

I heavily use my 400/5.6 for birding now and I keep it in my bag vs. the 300/4 which stays in my closet. It is a bit annoying when a bird gets too close and I can't focus on it, but otherwise it does a very good job. The 300/4 bare lens does AF faster, but that is probably more a case of the camera being able to use more focus points at the lower aperture. It is far quicker AF-wise than my 100-400 was.

I just used my 400/5.6 for only the 2nd time in the field for birding and was very pleased with the results. I was using my 7D on a tripod at ISO 400 and high shutter speed. These images are heavily cropped. I think the 400/5.6 is a great value compared to Canon's other super telephotos.

I am a complete newby to Photoshop as I just finished a series of video tutorials and used ACR, Photoshop for the first time on this set of images, so excuse me if I don't fully understand your questions. In ACR the crop % shows 54 for the seagull and 45 for the eagle. I believe I settled on Unsharp mask for the sharpening method. Some of the sharpness seems to have been lost in posting to this website so I have much to learn and improve upon!

I spent a lot of time debating this very choice. I shot with a 70-200 f4L IS on my 50D for a couple of years, with and without a 1.4ii extender. Loved the IQ of the naked lens, and the lens+1.4 was still very, very good, but it always seemed too short for my needs. I eventually opted for the 100-400 and I have been very pleased so far. Technique is crucial at the longer focal lengths, but when I nail focus at 1/60-1/125 (or use a good tripod) I think the IQ is just fantastic (unless shooting over ISO 800, of course, then noise prevails).

This funny shot of a suberb starling at the Atlanta Zoo was taken from about 12 feet at 400mm/f6.3 at 1/60 hand-held (200ISO) and minimally processed in Lightroom (basically just cut a portrait out of a landscape, so it's only cropped horizontally a bit, and bumped up clarity and optimized exposure). At 100% there's great detail in the feathers and the bokeh is really nice. I find this lens sharp wide open but it's hard to get the right DOF for many subjects so I bump to 6.3-8 more for DOF than lens performance. Even on this starling I'd like more of the beak in focus at 6.3, but it still works.

I was sorely tempted by the close focusing of the 300 f4 IS and AF of the 400 f5.6, but I wanted that 100-300 range too much. I think this is really splitting hairs and comes down to minor preferences until you get to options like the 300 f2.8L which is in another league (and price orbit).

I am a complete newby to Photoshop as I just finished a series of video tutorials and used ACR, Photoshop for the first time on this set of images, so excuse me if I don't fully understand your questions. In ACR the crop % shows 54 for the seagull and 45 for the eagle. I believe I settled on Unsharp mask for the sharpening method. Some of the sharpness seems to have been lost in posting to this website so I have much to learn and improve upon!

What we mean by 100% crop is that the actual pixels you see on the crop eg 1000x700 is the same as what that crop had in the original digital image (eg a 1000x700 section taken from the 5184x3456 for a 7D). You can use Photoshop to reduce the number of pixels, which is often done in the forums to give images that are smaller and load faster. Did you reduce the size of the image of the eagle as well as crop it?

I have rented both the 400 f/5.6L and the 300 f/4L IS. The 400 is sharper, the 300 gives a wider field and the convenience of IS...and a faster aperture. Neither is very sharp with a 2x TC, and I didn't try them with the 1.4x. Only the 300 would autofocus on any camera smaller than a 1 series, with a 1.4x TC attached.

Since you have the 100-400 already, I am betting you would miss the ability to zoom, more than you would like any small improvement in sharpness. You would also miss the IS.

If you have $3000 to spend, in addition to the funds you could get from selling the 100-400, you might consider buying a larger supertele, such as the 200 f/2, 300 f/2.8, and so on.

Perhaps save a little more mula ($$$) and purchase a used but nice EF 300mm f/2.8L IS lens. They are about $4000 in mint condition and work great with a 1.4x teleconverter and make for a really good 600mm f/8 ( stopped down 1 stop) lens.

I would stick with what you already have ( 100-400mm L ), if size and weight are a concern. You might find that you can wait even longer and maybe get a nice used 500mm f/4L IS.

300 f/4 IS + 2x ii TC (= 600mm f/8) + 1D Mark IV, center point only AF, hand held, ISO 1250, 1/1000, f/10. cropped 100% pixel to pixel. This is the sharpest shot I got with the 2x teleconverter added. Outside of the center image, weird wavy distortions were subtle (in other shots on the ground). They could be seen viewed at 100%...so I was surprised this shot was able to go as sharp as it did (which I admit isn't all that sharp). I used the "multiple half press" technique in single shot AF mode, to attempt the most accurate AF.